See this link for more details. The problem is that excess glycerin present in home brewed biodiesel can clog the fiters. Since glycerin is soap, it's basically soap clogging the exhaust filters. We will have to wait and see what impact the urea injection will have and if high percentages of commercially refined biodiesel will have. This is the same reason that bluetec is compatible only with the new ultra low sulphur diesel. (This is what almost all pumps have had for a while)
The big irony is that since biodiesel is so clean, the filters aren't as essential to producing clean emissions. The problem is when you use lower percentages of biodiesel or mostly fossil diesel.
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